At the moment when most social networks flatline, Facebook is taking off. But for Mark Zuckerberg, 175 million users are just the beginning of his headaches. Money, spam, and crime are spoiling his online utopia.

A rule of thumb Valley insiders use is that adding a million users to a site like Facebook requires $1 million in capital. That means Facebook should be spending roughly $5 million a week on servers and other computing infrastructure. From what's understood about its finances, Facebook is not covering the cost of its operations, let alone generating enough profit to pay for capital investments. The $360 million it raised from Microsoft and Hong Kong telecom mogul Li Ka-Shing in 2007 and 2008 will not last forever, especially at this pace.

And now is a terrible time to try and raise money. The social-networking investment bubble has decidedly popped. Friends Reunited, a social network purchased by British TV network ITV in 2005 for $250 million, is now worth as little as $30 million, analysts estimate. Facebook, likewise, is today worth a fraction of the $15 billion value at which it last raised money. It could well find new backers, but they will drive a harder bargain than Microsoft CEO Steve Ballmer.

What's Facebook really worth? The fast-growing social network is adding to its 150 million…
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The second reality: Spam.

Because of its privacy controls, clever algorithms, and draconian customer service, Facebook has proven relatively resistant to the kind of fake-hot-girl spam that ruined Friendster and MySpace. And when Facebook apps started getting spammy — zombies and pirates, anyone? —- Zuckerberg's social-network cops cracked down on those, too.

But hucksterism seem to be part of human nature. To the extent that Facebook seeks to reflect the real world, it's going to get filled up by people with something to sell. That these are actual people just makes it worse.

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Perhaps he can make friends with Leon Craig Ramsden, the man who reportedly posted on Facebook that he felt like "killin some1" before going to a bar and doing exactly that. If Facebook really aims to connect everyone in the world, it's going to get the Grahams and Ramsdens, too. How will parents feel about Facebook when their kids start getting friend requests from prison?

None of these obstacles are insurmountable. But they serve as a useful reminder: As Facebook grows up, things are just going to get less and less pleasant. Facebook began in the collegiate dreamland of Harvard, where the world's most entitled people gather to have their feelings of self-importance validated. After you leave campus, you enter the real world. Welcome, Facebook. Need a friend?